Nomad `host_network` documentation and example improvments

The Multi-Host Interface feature released in 0.12 can be tricky to configure properly as there are numerous different ways users can configure their host's network interfaces. It's also not clear from existing documentation how a `host_network` interfaces with some driver's networking features (ex. #8488).

The following documentation improvements are underway to help this.

* Updated examples in the docker driver documentation
* Multi-Host Interface learn guide
* New internals documentation on how to debug/understand how packets flow through Nomad's `bridge` networking mode, including the use of `host_network`.

Also cover #6873

Review the existing Docker driver documentation, the Multi-Host Interface learn guide, and issue #6873 first. Done means updated Docker examples, a clearer Multi-Host Interface guide, and internals documentation explaining how to debug packet flow through bridge networking with host_network.

Written by the indexing model from the issue text.